1. Field of the Invention
The present invention relates to a switching regulator and constant frequency compensating circuit, and more particularly, to a switching regulator and constant frequency compensating circuit for fixing an operating frequency according to an output voltage and a phase signal.
2. Description of the Prior Art
Power supply devices play an essential role in modern information technology. Among all the power supply devices, DC-DC (direct current to direct current) switching regulators have been widely used and are mainly utilized for providing stable DC power sources to electronic devices. Please refer to FIG. 1, which is a schematic diagram of a conventional DC-DC switching regulator 10. The DC-DC switching regulator 10 is utilized for providing a stable voltage to a load Load1, and includes an upper gate switch 100, a lower gate switch 102, a constant time trigger circuit 104, a comparator 106, an inductor L1, a capacitor C1, a reference voltage Vref1 and an inverter INV1. The constant time trigger circuit 104 outputs a pulse signal with constant active time to control operations of the upper gate switch 100 and the lower gate switch 102. Whenever the output voltage Vout1 is smaller than the reference voltage Vref1, the comparator 106 outputs a signal to the constant time trigger circuit 104, so that the constant time trigger circuit 104 outputs the pulse signal to turn on the upper gate switch 100 and turn off the lower gate switch 102. Thus, an external voltage source Vin1 supplies electric energy to the inductor L1 and then to the load Load1 via the upper gate switch 100. Since the pulse signal outputted by the constant time trigger circuit 104 has a constant active time, the upper gate switch 100 can be turned on for a constant interval when the output voltage Vout1 is smaller than the reference voltage Vref1. If the output voltage Vout1 is still higher than the reference voltage Vref1 after the upper gate switch 100 is turned on, the upper gate switch 100 would remain turned off until the output voltage Vout1 is smaller than the reference voltage Vref1. In other words, when the upper gate switch 100 is turned off, the output voltage of the DC-DC switching regulator 10 starts falling, and only when the output voltage Vout1 is smaller than the reference voltage Vref1, the upper gate switch 100 will be turned on again. In other words, the DC-DC switching regulator 10 utilizes a pulse width modulation (PWM) method to turn the upper gate switch 100 on/off, so as to regulate the power delivered to the load Load1 and thus to stabilize the output voltage Vout1.
Meanwhile, since the operating period of the PWM signal is the summation of the turn-on and turn-off periods of the upper gate switch 100, when resistance of the load Load1 varies, the duty cycle of the PWM signal varies accordingly, to stabilize the output voltage. In other words, since the turn-on period of the constant time trigger circuit 104 is fixed, and only the turn-off period can be changed, it implies the operating period (or the operating frequency) of the PWM signal varies as the resistance of the output load varies. Also, operating characteristics of some components in the DC-DC switching regulator 10, such as the inductor L and the capacitor C for enhancing energy efficiency and reducing ripples, are highly related to the operating frequency of the DC-DC switching regulator 10. In other words, if the operating frequency varies within a wide range, these frequency-sensitive components cannot be optimized. In such a situation, some negative phenomena occur. For example, the ripples of the output voltage Vout1 will become too large to meet requirements of some applications, because the operating frequency varies from the optimized frequency.
Please refer to FIG. 2, which is a schematic diagram of a conventional DC-DC switching regulator 20 for fixing an operating frequency. The structure of the DC-DC switching regulator 20 is different from that of the DC-DC switching regulator 10 by some components added for fixing the operating frequency. The DC-DC switching regulator 20 includes an upper gate switch 200, a lower gate switch 202, a constant time trigger circuit 204, a comparator 206, an inductor L2, a capacitor C2, a reference voltage Vref2a and an inverter INV2. Besides, the DC-DC switching regulator 20 further includes a frequency fixing circuit 250. The frequency fixing circuit 250 includes an error amplifier 252, a compensator 254, a frequency-to-voltage converter 256 and a voltage reference Vref2b. Noticeably, the constant time trigger circuit 204 further includes a control input terminal 204a and thus is not the same with the constant time trigger circuit 104. The control input terminal 204a is utilized for adjusting the turn-on period of the constant time trigger circuit 204, to fix the operating frequency. In other words, the turn-on period of the constant time trigger circuit 204 is no longer constant. In the DC-DC switching regulator 20, when the frequency tends to vary, the constant time trigger circuit 204 adjusts the length of the turn-on period according to the control signals received by the control input terminal 204a. In addition, the constant time trigger circuit 204 integrates the frequency to voltage converter 256, the error amplifier 252 and the compensator 254 into a closed control loop, such that the output voltage V256 of the frequency to voltage converter 256 can follow and fix at the reference voltage Vref2b. As a result, the operating frequency (or the operating period) of the PWM signal outputted by the constant time trigger circuit 204 can be fixed.
Since the operating frequency of the DC-DC switching regulator 20 can be fixed, the designer can optimize the designs of the frequency-sensitive components to reduce the ripples. However, in order to fix the operating frequency, the frequency to voltage converter 256, the error amplifier 252 and the compensator 254 of the DC-DC switching regulator 20 need to be implemented by more complex circuitry, causing larger chip area and higher production cost.